In the field of optical character recognition, an image containing characters is acquired and processed by special hardware and software that can identify the characters. For example, a camera may focus the image of an area of an object having the characters on some type of imaging array. The camera acquires an image of the character. The image is then processed by hardware and software which can identify the characters. The characters can be numbers, letters, special symbols or any other similar marking such as a bar code, for example.
Optical character recognition has use in many applications. For example, in the processing of semiconductor wafers an identifying string of characters such as letters and numbers, usually conforming to a standard character set and font, are etched onto the top wafer surface to serve as a unique identifier throughout further processing. It is very important to semiconductor manufacturers to be able to identify wafers during all processing steps. FIG. 1 shows a portion of semiconductor wafer 100 having a six character wafer ID 101. Often, as shown in FIG. 1, wafer ID 101 is placed near a flat 102 of the wafer. As is well known, wafer ID 101 may comprise a greater or lesser number of characters. The wafer ID 101 can be numbers, letters, special symbols or markings, or a combination of any or all of these.
In order for an optical character recognition system to read wafer ID 101, there must be sufficient contrast between the characters which make up wafer ID 101, for example, the character A shown as 101a in FIG. 1, and the surrounding region. In order for there to be contrast, the characters of wafer ID 101 must have different optical properties from the surrounding surface. In FIG. 1, region 140 is the area which will be focused by the camera on the imaging array and examined by the optical character recognition system. Field 140a is used to denote all of region 140 except for the characters of wafer ID 101. As is well known, the surface of semiconductor wafer 100 in the field 140a is highly reflective and is generally considered mirror-like. Therefore, light incident on field 140a reflects light specularly, and the angle of incidence equals the angle of reflection. The characters of wafer ID 101 must therefore either absorb light, reflect light in a different direction than light reflected from the field 140a or scatter light in order to generate contrast so that they can be read. In general, the wafer ID 101 is scribed on wafer 100 via laser. While the surface of wafer 100 in the region of the characters may remain highly reflective on the scale of the wavelength used to read the characters, the characters present an uneven surface so that incident light is reflected in many directions compared to light reflecting from the planar surface of wafer 100 in other regions. Additionally, while the Figures herein show scribe marks which have smooth, even surfaces which reflect light specularly at a different angle from the surface of wafer 100, at least some portions of the scribe mark are typically roughened on a microscopic scale, so that light incident on these portions is scattered in all directions.
There are many methods of scribing wafer ID 101 on semiconductor wafer 100. One method, referred to as hard mark or hard scribe, scribes a relatively deep trench (typically a few to several microns deep)into the surface of semiconductor wafer 100. Referring to FIG. 2A, a cross-sectional elevation view of semiconductor wafer 100 is shown. Scribe mark 201 could be for example a hard mark and forms a portion of one of the characters of wafer ID 101. Typically the characters of wafer ID 101 are made up of individual roughly circular scribe marks and thus are similar to dot matrix type characters. Alternatively, the characters of wafer ID 101 may be made of one or more long continuous trenches. As shown in FIG. 2A, scribe mark 201 has a kerr 202 surrounding the outer edges of trench 203. As can be seen, incident light 210 strikes the surface of wafer 100 at an angle of 90 degrees (perpendicularly). Since the surface of wafer 100 surrounding wafer ID 101 (i.e., the field 140a) is smooth and highly reflective, the angle of incidence equals the angle of reflection, and nearly all of incident light 210 is reflected straight back along the same path of the incident light rays 210 as shown by reflected light 211 (shown separated from incident light 210 for clarity).
Referring now to the incident light 210 striking the scribe mark 201 in the trench 203, it can be seen that the reflected light such as reflected light 215 and 218 is reflected in widely varying directions, depending upon the portion of scribe mark 201 which it hits. Typically, the angle of incidence and angle of reflection are measured from the normal to the surface. However, for clarity in the drawings, the complements of these angles are shown and described in the Figures and accompanying text herein. For reflected light 215, for example, the angle of reflection 217 equals the angle of incidence 216 at the point on the wafer where incident light 210 strikes. However, this surface is at an angle compared to the plane of the surface of wafer 100 and field 140a. Therefore, while some light may be directed straight back up in the direction of incident rays 210, a significant fraction such as reflected light 215 and 218, is reflected in other directions. In addition, because the surface of wafer 100 is typically roughened in at least some portions of the character, much of the incident light is scattered in all directions. Additionally, the surface characteristics may be changed such that a greater portion of incident light 210 incident on the characters is absorbed compared with the field 140a. When looked at from directly above, these differences in physical properties between the field 140a and the scribe mark 201 have the effect of making the field 140a look bright, and the scribe marks, and therefore the characters, look dark. This type of illumination is known as bright field illumination.
FIG. 2B shows the same cross-sectional view in FIG. 2B, however, incident light rays 220 strike the field 140a at an angle 222 of less than 90.degree.. The reflected light 221 leaves the surface at an angle 223 which, because the surface is highly reflective and smooth, is equal to the angle of incidence 222. Again, some of incident light 220 is shown striking the surface of trench 203. The angle of reflection 227 for the reflected light 225 is equal to the angle of incidence 226 at the point the light strikes in trench 203 for some of incident light 220. Because the surface in scribe mark 201 is at an angle relative to the plane of the surface of wafer 100, very little of the incident radiation 220 in the region of scribe mark 201 is reflected in the direction of reflected light 221 from the field 140a. Again, as with FIG. 2A, FIG. 2B shows a smooth surface in the region of scribe mark 201. However, the surface on a microscopic scale may be very rough or faceted. This still has the same effect as discussed in relation to FIG. 2A, namely, that the incident radiation in the scribe mark 201 gets reflected and/or scattered in all directions, with only a small, random portion being reflected in the direction of reflected light from the field 140a. In general, with marks such as scribe mark 201, having large features out of the plane of the field 140a such as kerf 202 and trench 203, much of the light is reflected at high angles relative to the plane of the field 140a. Stated in another way, with marks such as scribe mark 201, it can be expected that at a large angle away from the angle of reflection of the field 140a, where little or no light reflected from the field 140a will be detected, there will be a significant amount of reflected/scattered radiation detected from the scribe mark 201.
FIG. 2C shows the same cross-sectional view as FIGS. 2A and 2B. In FIG. 2C, incident light 230 strikes the field and the scribe mark 201 at a very glancing angle 232. Reflected light 231 from the field 140a is reflected at an equal angle of reflection 233. Again, as with FIG. 2A and 2B, much of incident light 230 striking the scribe mark 201 is reflected at a high angle relative to the plane of the field 140a. Because light is reflected off of scribe mark 201 at a high angle, when the region 140 is illuminated with incident light 230 at glancing angle 232, much of the light is scattered or reflected upward. Therefore, if a detector is placed, for example, directly above the region 140, the field 140a will appear dark (since most reflected light 231 is reflected in direction 231 ). The characters will appear bright due to the light reflected and/or scattered upward. This method of illumination is known as dark field illumination.
FIG. 3 shows two of scribe marks 301 on semiconductor wafer 100. Scribe marks 301 could be, for example, soft scribe marks. As can be seen from the Figure, the trenches 303 of the scribe marks 301 are shallower than for the mark 201 of FIG. 2. Additionally the kerf 302 is smaller. Incident light 310 strikes the field 140a of semiconductor wafer 100 at an angle 312 relative to the surface of the field 140a. Reflected light 311 from the field 140a is reflected at an angle of incidence 312. Light reflected from the scribe marks 301, such as reflected light 315 and 316 is shown together with arrows 31la, which show the direction of reflected light 311 from the field 140a. As can be seen, much of the light reflected from the scribe marks 301 such as 315 and 316, is reflected at a relatively small angle to the direction 311a. In general, shallow marks such as scribe mark 301 scatter or reflect light at a small angle relative to reflected light 311. Therefore, most of the light reflected off of scribe marks 301 will be concentrated at a small angle away from the direction of reflected light 311 from the field 140a.
The above described difference between the scribe marks 201 of FIG. 2 and scribe marks 301 of FIG. 3 creates a problem for optical character recognition systems. For example, referring to FIG. 2A, the method often used to read such characters is the bright field illumination described above. In this method incident light is directed at the characters from directly above such as incident light 210. A detector, such as a camera is placed directly above the characters. The incident light 210 is typically a collimated light beam which is injected into the optical path coaxially by use of a beam splitter. The illumination is referred to as coaxial because the illuminating light travels the same path as the reflected light which is detected. As described above, the field 140a reflects all incident radiation back, while scribe marks 201, which make up the characters, reflect in all directions. This gives rise to good contrast allowing the characters to be easily read. However, referring again to FIG. 3, as mentioned, the scribe marks 301 do not give rise to a significant degree of scattering. Because of this, if a coaxial bright field beam is used, the field will appear only slightly brighter than the characters. That is, the contrast is relatively poor. Thus the coaxial method used for reading a character with scribe marks such as scribe mark 201 cannot be used to read a character composed of scribe marks such as scribe mark 301.
Another problem encountered in optical character recognition is that some processing, such as an edge bead removal process can present lines and contrasts which are not part of the characters. Referring to FIG. 1, markings 125 and 130 are shown. Generally the markings from an edge bead removal process look like rings or bands as shown. In addition, there is often discoloration or regions of differing color between the rings, for example between ring 125 and 130. The markings from the edge bead removal process do not appear to give rise to much scattering. However, the markings such as line 125 and 130 have a different reflectivity than the surrounding surface and can confuse the optical character recognition process. For example, line 130 may make the C in wafer ID 101 look like an E when observed with bright field illumination.
A further problem with optical character recognition illumination is that light from the environment in which the optical character recognition is taking place illuminates the wafer from all directions. This tends to reduce contrast in most cases.
What is needed is a single system which can provide illumination such that all types of scribe marks can be read. Further, the system should be able to read scribe marks which have partially obscured characters due to, for example, markings due to edge bead removal processes. The system should have a reduced sensitivity to environmental lighting. Finally, the system should not require costly and complicated optics such as lenses, beam splitters, etc.